The present invention concerns an improved process for manufacturing 1,4-bis (phenylglyoxyloyl) benzene and 1,3-bis (phenylglyoxyloyl) benzene, which constitute monomers for the manufacture of polyphenylquinoxaline resins.
It is known, in the present state of the art, that the polyphenylquinoxaline resins have exceptional properties of heat stability and resistance to oxidation and to chemical agents, but that an extended use thereof is limited by the excessive cost of the starting monomers, particularly the aromatic bis-.alpha.-diketones. The manufacture of these compounds is effectively based on the use of rare and expensive reactants and requires many intermediary steps.
It is also known that the best way to obtain the aromatic .alpha.-diketones is to oxidize the corresponding .alpha.-keto alcohols, called benzoins. As a rule, benzoins are prepared by condensing one or more aromatic aldehydes in the presence of an alkali metal cyanide, used in an amount far lower than the stoichiometric amount. It has however been stated recently (Kratzer and coll., J. Org. Chem. 41, (1976), 2230) that the mixed benzoinic condensation of terephthalaldehyde with benzaldehyde does not yield the expected bis-benzoin. This compound could only be obtained by first preparing the bis-bisulfitic addition product of terephthalic aldehyde, then reacting it with potassium cyanide in a molar proportion of two, to synthesize the terephthalic bis-cyanohydrin, and finally condensing the latter with a large excess of benzaldehyde. It appears that this method has the disadvantage, as compared with the benzoinic condensation, to make use of potassium cyanide in large amount and to necessitate several distinct reaction steps.